The invention relates to a process for the finish-machining of the bearing positions on main bearing journals and connecting rod bearing journals of crankshafts for motor car engines, whereby the crankshafts have roundings between the bearing positions and the transitions adjacent in each case to the bearing positions, such as for example cheeks or adapting bearings.
To increase the fatigue strength of the crankshaft of engines for motor vehicles, the transitions at the bearing positions of the main bearing journals and connecting rod bearing journals are deep rolled. In this situation, deep rolling rollers, which have a diameter of some 15 mm and a rounding radius of some 1.3 mm, are pressed with defined force into the radii or recesses which delimit the individual bearing position of the main bearing journals or connecting rod bearing journals on both sides. As a result of the pressing of the hard deep rolling rollers, plastic deformation occurs between the transition, such as for example the cheek or the adapting bearing of the crankshaft, and the bearing position, and in this way induces a state of pressure internal stresses into the crankshaft, which increases the fatigue strength of the crankshaft. In this situation, a part of the width of the bearing position is required for the deep rolling. From a theoretical maximum available width between two adjacent transitions and the bearing position of a main bearing journal or a connecting rod bearing journal relating to them there is accordingly only a reduced width available as a contact area for the connecting rod or the main bearing. As a result of the higher degree of exploitation of engines, and of diesel engines in particular, there is a desire to be able to make use of the largest possible width of the bearing position at the main bearing journal and connecting rod bearing journal. It is true that the usable bearing width increases as the rounding radius of the deep rolling roller decreases, but at the same time the fatigue strength of the crankshaft attainable with deep rolling is reduced.
In particular, recesses or fillets increase the level of tension on crankshafts under flexural and torsion stress, because at the same time they weaken the diameter at the transition to the cheek. This applies equally to main bearings and stroke bearings, so that, as a result of the recesses, the stress at the transition to the cheek is further increased.
From DE 198 33 363 A1 a “method for the lathe machining of rotation surfaces on workpieces, preferably on crankshafts, and a disk-shaped tool for carrying out the method” is known. For the lathe machining of crankshafts provision is made for a disk-shaped tool, which consists of a centering body, a carrier body, and several tool units for dressing the free recesses on the main bearing journals and connecting rod bearing journals, as well as other tool units for dressing the actual bearing positions on the main bearing journals and connecting rod bearing journals, located between the free recesses. In this situation, the free recesses are dressed by several tool units, each of which contains a dress-machining cutting insert. By contrast with the preliminary machining of the bearing positions by roughing, in this situation the fine machining of the recesses and bearing positions is carried out in a known manner by dressing. Accordingly, a good surface quality and low peak-to-valley roughness is achieved at the machining positions, as a result of which the fatigue strength of the crankshaft is increased.
A method, tool, and device for material-removing machining of crankshafts are further known from European Patent Application EP 1 052 049 A2. According to this, it can be advantageous in mechanical engineering terms if, during the manufacture of crankshafts, the bearing zones are in each case milled in the first machining step, after which, in the second machining step in each case the rotational movement of the tool ends in a predetermined position, and in this position the roundings or fillets of the bearing journals arranged on both sides of the bearing surface are lathe-machined or turned. Accordingly, the edges which are required for technical application reasons on dynamically highly-stressed parts are rounded or prepared with fillets, which then lead to an increase in the fatigue strength of the crankshaft.